Abstract
In the cargo tank structure of the chemical tanker, the cruciform welded joints between the trans-verse bulkhead and the double bottom requires the most attention to prevent fatigue crack initiation because of its severe stress condition. By the way, laser scanning measuring instruments are becom-ing smaller and lighter and handheld types are being commercialized.
We have studied practical application of a technology to estimate stress concentration factors by measuring the shape of welds in cargo tank structure of chemical tanker with a weld bead laser scanner during construction at a shipbuilding site.
In this study, the welded joint surface shapes of fatigue test specimens simulated as the cruciform welded joints between the transverse bulkhead and the double bottom were measured by a weld bead laser scanner and the stress concentration factors (KtL) of the welded joint of the specimens were calculated by using the conventional estimating equation with the measured geometrical data of the welded parts. In addition, the static FEM analysis was also conducted to obtain the stress concen-tration factors (KtF) using the same measured data. Comparing the stress concentration factors KtL and KtF with the stress concentration factor KtS calculated by the conventional estimating method with silicon mold, it was shown that KtL and KtF values were about the same as that of KtS. In addi-tion, the representative value of the fatigue strength at 2×106 cycles for cruciform welded joint was derived considering the fatigue test results using the cruciform welded joint specimens and the re-sults of previous studies, the allowable stress concentration factor (KtA) was presumed based on the representative value of fatigue strength and design S-N curve for cruciform welded joints. As a re-sult, it is proposed that the guarantee which fatigue crack initiation does not occur at the toe of welded joint can be valid when the geometry of weld joints in the cargo tank structure under con-struction at shipbuilding site is measured by using the weld bead laser scanner and immediately the stress concentration factor KtL or KtF is calculated and the value is evaluated less than KtA.
This is a revolutionary technique in terms of quality control and assurance of shipbuilding.
